Have you ever wondered if that one mutant woopa might just be your lucky shot at getting slam II? Of course, you might say that case would be nearly impossible because of the drop rate for slam II. But how exactly do the mechanics of "drop rate" work? Drop rates aren't really explained in the game but I have thought of some theories of my own as to how drop rates might work in twom.
Note: None of what I state is actually true. These are merely "What if" theories. Drop rates might best be explained with the "Dice Thoery" where everything is dropped at random chance.
This theory suggests that each monster is programmed to drop random gold and items at fixed rates. This is the main outlook on drop rates. It's as simple as rolling a dice: while the monster is spawned, the dice is rolling. When the monster is killed, the dice has stopped and what you see is what you get. As for the "fixed rates" part, programmers might've integrated a percent chance to drop certain items. For example, some people might say that the drop rate for slam II is 1 out of 100 mutant woopa kills. Programmers could've added that percentile into each mutant woopa. This is the generally accepted theory as to how drop rates work.
This theory suggests that each monster already has the amount of gold and items to be dropped once killed. Look at it this way: You have a chief woopa roopa pinata and your friend stuffed it with a certain amount of gold and a totem. After you have broken the chief pinata, the goodies inside are revealed to you. What if each monster in twom is like a pinata where the "goodies" are already inside them, just waiting to be opened?
This theory suggests that the gold and items dropped would be determined by the "tile" it dies upon. By tile, I mean a certain point in the map or in other words, the coordinates of the map. Suppose you kill a monster at a certain point in the map and you get good drops. What if each point in the graph is the key to what items you'll get from the monster? Programmers might've made it possible so that whenever a monster is killed at point (1000,3450) in a certain map, you would get the good drops from that monster. If you kill a monster in point (550,2370), however, you would only get gold. If this were true, then killing monsters would be the same as digging for treasure in the maps of twom.